The total energy generated by 1 mole of pyruvic acid is 8 ATP. After glycolysis and
TCA, a net total of 38 ATP would have been produced. After glycolysis and TCA, a net
total of 38 ATP would have been produced.
Reaction catalyzed Method of P
Reaction catalysed Phosporylation
substrate level GTP
No of ATP formed
per mole of
Series of reactions that occurs in the body are called pathways. They are processes by
which the cell regulates itself, various myriad of enzymes that catalyse chemical
reactions that take place in living celsl are referred to as metabolism.
Metabolism therefore is the totality of chemical reactios in living matter. Metabolism has
Metabolism obtain chemical energy from degradation of energy nutrients
from the environment or from captured solar energy.
To convert nutrient molecules into building block precursors of cell macromolecules.
To assemble this building block into protein, nucleic acid, lipid,
polysaccharides and other cell components.
To form and degrade bio-molecules required in specialized functiosn of cell.
Metabolism consists of 2 major pathways namely:
Catabolic (degradation) pathway
Anabolic (Biosynthesis) pathway
Anabolic takes place in 3 stages beginning with small precursors. Acetyl CoA are built
up with fatty acid which are in turn assembled to form lipids.
Catabolism is a converging process, since it begins with few simple precursors from
which large varieties are made.
REGULATION OF METABOLIC PATHWAY
The regulation of metabolic pathway is brought about by 3 different types of
Through the allosteric enzyme
By hormonal regulation
By concentration of a given enzyme on the cell.
Glycerol & fatty acid
Acetyl CoA (TCA)
enters the TCA cycle
Catabolism of lipids is divided into 3 stages.
In stage 1, hundreds of lipids are broken down into their building blocks which are few in
number. Lipid in most organisms are in the form of tri-acyl glycerol. The term fat refers
to this most abundant class of lipids. Triacyl glycerol plays no other role than energy
storage. Most of the energy derived from fat comes from the oxidation of the constituent
fatty acids. The brain is the only tissue that is unable to use fatty acid as a significant
energy supply. However, under conditions of starvation and long fasting, the brain can
adjust to use lipid related compounds like ketone bodies.
In stage 2, the building block molecules are further degraded into common products
acetyl CoA and pyruvate.
In stage 3, the cell catabolism converge into the TCA cycle with the formation of 2 major
products water and CO2.
FAT DIGESTION AND ABSORPTION
The product of fat digestion is a mixture of free fatty acids (FFA), mono and diacyl
glycerol. During the absorption through the mucosa cells, some of these products from
hydrolysis condense to re-synthesize triacyl glycerol. This is known because much of the
absorbed lipids is in form of triacyl glycerol complexed with protein, sometimes it can
form complexes with cholesterol and sometimes complexes with phospholipids. When it
forms complex with protein, it is called lipoprotein. When it forms complex with
cholesterol, we call it chylomicrons or portomicrons.
Fats are derived from 2 primary sources in the body;
Mobilization of fat stored in the adipocytes.
OVERVIEW OF FAT DIGESTION AND ABSORPTION
through blood to
After absorption of lipids, it can be associated with protein to form lipoprotein which aid
in the transportation of lipid for energy storage. The lipoprotein are synthesized mainly in
the liver. Lipoproteins are classified according to their density into;
Very low lipoprotein
High density lipoprotein
very high density lipoprotein
MOBILIZATION OF FAT
The release of energy stored in the fat is comparable to the mobilization of carbohydrate
stored in animal glycogen. The steps involved are;
STEP I: Activation of adenylate cyclase ephinepherine.
STEP II: Active adenylate cyclase in turn activates protein kinase by phosphorylation.
STEP III: Active protein kinase activates an enzyme called triacyl glycerol lipase.
STEP IV: This enzyme catalyses hydrolytic released of fatty acid from carbon 1 or 3 of
STEP V: The fatty acid released is followed by the action of a di and mono acyl glycerol
lipase. These three enzymes degrade the original molecule to glycerol and 3 un-esterified
The hormones are mediated by epinepherine in stress situation and it is also mediated by
glycogen during fast.
FATTY ACID OXIDATION OF PALMITYl CoA
CH2- O – C – R1
CH - O – C – R2
CH2 - O – C – R3
CH3 – (CH2)11 –CH2 ---CH2 –CH2—C
CH3 – (CH2)11 –CH2 ---CH2 –CH2—C
CH3 – (CH2)11 –CH2 ---CH2 –CH2—C
1-2 hydroxyl CoA
CH3 – (CH2)11 –CH2 ---C – S ---CoA -- CH2—C
3 Ketoacyl CoA
7CH3 --- C
Palmitoyl CoA + 7 CoA – SH + 7 FAD + 7NAD + 7H2O
7FADH2 + 7NADH + & H
Activation of Palmitoyl CoA
Oxidation of 8 Acetyl CoA 8 x 12
Oxidation of 7 FADH2
Oxidation of 7 NADH
CHAINS OF KETOGENSIS
C -- S --- CoA
C ---- CH3--- C ---- S ---- CoA
β hydroxyl butyrate dehydrogenation
β hydroxyl butyrate
When FADH is been dehydrogenated, it will be converted to flavine adenine dinucleotide
hydrogenase likewise NAD.
NOTE: Acetyl CoA is 8 times because molecule of fats gives 130 ATP that is why fatty
acid gives a higher energy than carbohydrate.
Catabolism of fats begins with hydrolysis of triacylglycerol to yield glycerol + 3
molecules of fat comes from fatty acids with 5% of glycerol. Most of the fatty acids arise
in the cytosol, either through biosynthesis or through triacylglycerol transported from fats
depot outside the cell. There is a formation of acyl CoA made up of long chain of fatty
acids instead of 2 carbon acetic acids. The endoplasmic recticulum at the outer
membrance of mitochondria uses this as fuel within the mitochondria, these reactions are
catalysed by carnitine acyl transferase which transverse within the inner membrane. The
oxidation of fatty acids is a series of steps in which each step release 2 carbon fragments
in form of acyl CoA the pathway is cyclic which involves 4 reactions and ends with the
formation of acyl CoA short ended by 2 carbon which undergoes the same process in the
e.g. Palmitoyl CoA undergoes 7 cycles of oxidation to form 8 molecules of acetyl CoA.
Each cycle release 1 -2 carbon units with 2 electrons.
Oxidation of saturated fatty acyl CoA involves the following reactions:
dehydrogenation to form Enol CoA
Hydration of the double bond forming 1,3 hydroacyl CoA.
Dehydrogenation of the hydroxyl group to form 3 ketone CoA
Cleavage of the 2nd molecule co-enzyme to release acyl CoA.
Acetyl CoA has 2 metabolic fates;
Oxidation to CO2 in the TCA cycle
Biosynthesis of fatty acid
Another major pathway comes into play in the mitochondria when acetyl CoA
accumulate beyond its capacity to be oxidized or used for fatty acid synthesis. That
pathway is called ketogenesis and it leads to a class of compounds called Keton bodies.
Foods are needed for synthesis of products in the body. They are needed as source of
energy. Foods are involved in the transfer of energy such as when chemical energy is
converted to mechanical energy of heat. The ability of a food to supply energy is
therefore of great importance in determining its nutritive value.
Energy is needed for mechanical work (i.e muscular activities), for chemical works such
as movement of substance against concentration gradients, synthesis of enzymes and
When the chemical energy of the food is used for muscular and chemical work involved
in body maintenance, the animal does not work on its surroundings and the energy used is
converted to heat. In fasting animal, the quantity of heat produced is equal to the energy
of the tissues catabolised. Energy supplied by the food in excess of that needed for
maintenance is used for various forms of production. A young growing animal will store
energy principally in the protein of its new tissues whereas an adult will store relatively
large energy in fat. Whereas,a lactating animal will transfer food energy into energy
contained in milk constituent.
TYPES OF ENERGY
Heat Energy: This is the energy produced only to keep the body warm. Its
production is taken into account in measuring the efficiency of body
processes. The basic unit of energy is called Calorie (Cal). A calorie is defined
as the amount of heat energy required to raise 1g of water from 14.5oC to
15.5oC. This is small, hence in practical nutrition a large calorie is used i.e
Calorie (Cal) is defined as the amount of heat energy required to raise 1 kg of
water by 1oC.
Gross Energy: When a substance is completely burnt to its ultimate oxidation
product i.e CO2 and water and other gases, heat is given off and that heat is
referred to as Gross energy. The quantity of heat resulting from complete
oxidation of a unit weight of food is known as Gross energy. Gross Energy
(G.E) is sometimes called heat of combustion.
The oxidation of hydrogen and carbon atom in food leads to gross energy.
CH2O have enough oxygen in the molecule to oxidize the hydrogen. Hence
the only produced heat is from the oxidation of carbon.
In fats, less oxygen is present in the molecule hence heat is produced from the
oxidation of carbon and hydrogen. This gives the reason why the heat
produced in fat is higher than CH2O.
In protein, heat is generated also from the combustion of carbon and hydrogen
but the nitrogen that is present in protein gives rise to no heat at all since it is
set free as gaseous form. The gross energy value of the 3 classes of feed
ingredients is stated below;
CH2O = 4.15Kcal/g
Protein = 5.65Kcal/g
Fat = 9.4 Kcal/g
Digestible Energy: Apparent digestible energy of a food is the gross energy
content of a weight of the food less the gross energy content of feaces
resulting from the consumption of a unit weight of that food. Apparent
digestible energy is the energy generated from the food minus the energy
generated from the feaces.
Metabolisable Energy (M.E): M.E of a food is the digestible energy less the
energy lost in the urine less the energy lost in combustible gases. Energy in
urine is present in nitrogen containing substances such as urea, allantoin, uric
acid, hippuric acid, creatinine, glucuronate. The combustible gases lost from
rumen consist of methane. Methane production is directly proportional to food
At maintenance level of nutrition, 7-9% of G.E is lost as methane. For
poultry, metabolisable energy is measured easily than digestible energy. Why?
This is because in poultry the feaces and urine are mixed together so the M.E
can be easily measured during the process.